This invention relates to magnetic sensors, and particularly to magnetic sensors used to sense positional changes in a moving object.
According to the well-known Hall effect, if a current is provided across a conducting or semiconducting plate along a first axis in a plane of the plate and a magnetic field is applied along a second axis perpendicular to the plate, an electric field (and hence a voltage) is induced in the plane of the plate along a third axis that is mutually orthogonal to the first and second axes. The induced voltage is proportional to the sensed magnetic field, and thus the sensor can be used to sense the magnitude of the magnetic field, or it can at least be used to sense when changes occur in the magnitude of the magnetic field.
One known application for a magnetic sensor based on the Hall effect is described, for example, in Wolf, U.S. Pat. No. 4,970,463. Referring to FIG. 1 herein, a magnetic field sensor 10 is positioned between a source 12 of a magnetic field (such as a permanent magnet), and a ferrous object 14, such as a gear with teeth that effectively define larger radius portions 16 and smaller radius portions 18. As object 14 rotates about an axis 20 and the larger and smaller radius portions 16 and 18 pass across sensor 10, the distance between source 12 and the outer boundary of object 14 effectively changes, thereby causing the magnetic field between them and hence the induced voltage to change. Circuitry coupled to the sensor is used to sense such changes that exceed a desired threshold, thereby indicating the occurrence of a transition when an edge of a gear tooth passes the sensor.
Two or more magnetic field sensing elements 22a, 22b can be spaced apart in one sensor so that a difference signal can be taken. Methods for using signals from three and four sensing elements arranged in a straight line are described, for example, in U.S. Pat. No. 4,737,710. With such multiple sensing elements the elements need to be properly aligned relative to the position and rotation of the shaft. Otherwise, a sensing element may not be in position to sense changes in the field.